perf(layout): track panel resizing at a stepped cadence

apps/desktop/src/composables/useTabScroll.ts

@@ -1,4 +1,5 @@
 import { onBeforeUnmount, ref, watch, type Ref } from "vue";
+import { deferUntilPanelResizeEnd } from "@/lib/app/panelResizeState";
 
 export function useTabScroll(tabsContainerRef: Ref<HTMLElement | null>) {
   const hasTabOverflow = ref(false);
@@ -20,6 +21,10 @@ export function useTabScroll(tabsContainerRef: Ref<HTMLElement | null>) {
       scrollThumbWidthPercent.value = 100;
       return;
     }
+    // Skip DOM geometry reads while a divider drag is in flight; they force a
+    // synchronous relayout of the whole document on every frame. Re-measure
+    // once when the drag finishes.
+    if (deferUntilPanelResizeEnd(updateScrollButtons)) return;
     const maxScrollLeft = Math.max(0, el.scrollWidth - el.clientWidth);
     hasTabOverflow.value = maxScrollLeft > 1;
     const rawThumbWidth = el.scrollWidth > 0 ? (el.clientWidth / el.scrollWidth) * 100 : 100;